An election law introduced by Silvio Berlusconi could result in deadlock between the upper and lower houses of parliament. Photograph: Alessandra Benedetti/Corbis

Why should I care what happens? Just that the wrong result could reignite the eurozone crisis, affecting the world economy.

Why? Italy has huge public debts, amounting to over 120% of its GDP. If investors lost confidence in its ability, or willingness, to repay them, it would trigger panic on the bond markets where government debt is traded. That could happen in one of two ways: if investors felt either that the Italians had picked a government that could lose its grip on the budget (since budget deficits add to the debt) or one that would not introduce the reforms the economy needs to grow.

In other words, one led by Silvio Berlusconi? He does have previous. The public accounts spiralled out of control when he was in office from 2001 to 2006. During the 2000s, when he was in power for most of the time, the only countries with worse growth records than Italy were Haiti and Zimbabwe. It was market concern over his reluctance to impose painful reforms that took the eurozone to the brink of disaster in November 2011. Berlusconi stepped down to make way for a non-party government headed by Mario Monti, which has been in office ever since.

So a Berlusconi victory would be the 'wrong' result? Not the only one. Even worse would be no government at all.

How could that happen? The Italian parliament has two chambers and the upper house, the senate, has exactly the same powers as the lower house, the chamber of deputies. So if a party – or alliance of parties – won only one of the two houses, it could not pass laws.

But that's unthinkable … Unfortunately not. Because of an election law brought in by – right first time – Silvio Berlusconi, the rules for the two chambers are different. In the lower house, the alliance that comes first gets bonus seats to ensure it has a clear majority. But, in the senate, the bonus seats are doled out to whichever alliance comes first in each of Italy's 20 regions.

So there could be a majority for one group in the chamber of deputies …

And for another in the senate.

Nasty. But likely? Possible. The centre-left Democratic party and its allies started the campaign with a clear lead. But Berlusconi's alliance of his own Freedom People movement and the Northern League is catching up. At the same time, the polls show the Five Star Movement, fronted by a comedian, Beppe Grillo, overtaking the alliance that backs Monti.

Why does that matter? Grillo says his movement will not go into government with any of the mainstream parties.

Wow! I can't wait for the next poll

Dream on. Publication of opinion polls is banned in the last two weeks of campaigning.